1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138 U.S.A., sunnyp@caltech.edu, ishii@eps.harvard.edu
2. Now at Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Boulevard, Seismological Laboratory 252‐21, Pasadena, California 91125 U.S.A.